1. Field of the Invention
The present invention relates to a connecting apparatus such as a thermocompression bonding apparatus, which is used for, e.g., connecting an electrode substrate of a liquid crystal display and a printed circuit board mounted with electronic parts.
2. Description of the Related Art
Utilizing the features characterized by thinness, light weight and low electricity consumption, a flat panel display, represented by a liquid crystal display, has been recently used widely in such applications as display for personal computer, word processor, television and the like or as display for projection.
Among the flat panel displays, an active matrix type liquid crystal display, which comprises pixel electrodes and switching elements electrically connected thereto, has become the target of intensive research and development, because such display provides a good display image free of cross talks between the adjacent pixels.
Generally, an active matrix type liquid crystal display has an array substrate and an opposing substrate, which are positioned opposite to each other, and a liquid crystal composition is filled in the space between the substrates via an orientation layer. The array substrate has a glass substrate on which a plurality of signal lines and a plurality of scanning lines are arranged in a matrix and pixel electrodes are positioned via thin film transistors (hereinafter referred to as TFT), functioning as switching elements, in the neighborhood of each of the cross sections between the signal lines and the scanning lines. On the glass substrate, storage capacitor lines are provided in parallel to the scanning lines and insulating layers are provided between the scanning lines and the pixel electrodes so that storage capacitor is formed between the storage capacitor lines and the pixel electrodes.
The counter substrate has a glass substrate on which a matrix-shaped shading layer for shading the TFTs and the neighborhood of the pixel electrodes is formed, and a transparent counter electrode is positioned on the shading layer via an insulating layer.
Each of the signal lines and each of the scanning lines on the array substrate are electrically connected to a drive circuit board, which drives or controls the display panel, through a tape carrier package (hereinafter referred to as TCP) which comprises a driving element mounted on a flexible printed circuit board. The flexible printed circuit board has an insulating sheet formed of a heat-resistant material such as polyimide, and conductive lines formed on the insulating sheet.
Generally, the connection between the array substrate and the TCPs is made by the thermocompression bonding by use of a thermocompression bonding apparatus while an anisotropic conductive film that has a bonding resin containing electroconductive particle dispersed therein is sandwiched between the array substrate and the TCP.
The thermocompressing apparatus is equipped with a thermocompressing head which, while kept in a heated state, presses the lead portion of the TCP from above. Generally, the thermocompression head is equipped with a U-shaped heater tool comprising a pair of parallel legs and a distal end portion that couples the two legs at one ends. The distal end portion of the heater tool is pressed against the end portion of the TCP and the array substrate in a parallel manner.
In order to assure the compression bonding of all of many outer leads of the TCP to the electrodes of the array substrate by means of the above-mentioned thermocompressing head, it is necessary to uniformly press the outer leads of the TCP by means of the heater tool. That is, where the distal end portion of the heater tool has an inclination to the array substrate, part of the outer leads is not sufficiently pressed by the heater tool and, as a result, a connection failure can occur. The degree of parallel of the distal end portion of the heater tool may change at the time of exchange of the heater tool and, in addition, it may vary with time during the use of the thermocompression bonding apparatus.
Therefore, generally, the thermocompressing bonding apparatus has an adjusting mechanism for adjusting the degree of parallel of the distal end portion of the heater tool. The adjusting mechanism has a construction that a heater tool is attached to a support block which is rotatably mounted on a base so that the degree of parallel of the heater tool may be adjusted by rotating the heater tool together with the support block.
Heretofore, the constructions, which rotatably hold the support block, include a construction that utilizes a screw as the rotation shaft for the support block, a construction that holds the rotation shaft of the support block via bearing, and a construction in which the support block and the base each have a spherical support surface so that the spherical surfaces are slidably engaged with each other.
However, the support mechanism that utilizes a screw as a rotation shaft has looseness in the tapped hole which is formed in the support block and therefore the degree of parallel of the distal end portion of the heater tool tend to vary. And, the support mechanism that utilizes bearings tends to cause the variation of the degree of parallel due to the looseness of the bearings. Accordingly, a lot of time is necessary for the adjustment of the degree of parallel with a high-level of precision by means of the above-mentioned adjusting mechanism.
Meanwhile, in the adjusting mechanism that has spherical support surfaces, the problem is that the spherical surface preparation requires a high-level precision and it is very difficult to prepare a looseness-free support surface. As a result, the production cost is high.
Further, as a conventional adjustment mechanism, there is provided a mechanism wherein a movable portion of a micrometer is secured to a support block and the rotating amount of the support block is adjusted by means of the micrometer. However, the problem is that, since a large load is impressed on the micrometer at the time of adjusting operation, the precisely created screw threads easily deform thereby making it difficult to carry out highly precise adjustment.
Recently, a folded TCP has been increasingly used. The folded TCP has relatively long distance between the driving element and the outer lead ends and incorporated into a liquid crystal display while part of the flexible printed circuit board is in a folded state. In this folded TCP, the end portion of the flexible printed circuit board deforms three-dimensionally, for example in the shape of wave, depending on the case.
When the wave-like end portion of the TCP are superimposed over the predetermined location of the array substrate, the outer leads of the TCP do not make a uniform contact with the array substrate thereby producing partial lifting. Therefore, when the deformed end portion is subjected to the thermocompressing bonding treatment by means of the heat tool, the lifted portion is stretched owing to the pressing action of the heat tool thereby causing location error from the array substrate. As a result, it is difficult to precisely connect each of the outer leads to the corresponding electrode on the array substrate.
On the other hand, when carrying out the thermocompression bonding of the outer lead portion of the TCP to the electrodes on a circuit board by use of solder, there is a risk that connection failure may occur due to the lifting of the outer lead portion, because the heater tool is pulled up while the solder is in a molten state after the heater tool is pressed against the outer lead portion.